Early life stress impairs hippocampal development across diverse mammalian species, including rodents, suggesting that animal models may help elucidate the molecular and cellular changes that guide these developmental changes in children. We developed a mouse model of early life stress known as brief daily separation (BDS) that recapitulates several clinical features seen in maltreated children, including abnormal hippocampal function. Hippocampal deficits are detected in juvenile male BDS mice, but not in female littermates. Here we propose that higher levels of corticosterone in male pups exposed to BDS lead to sex- specific reduction in the levels of the master regulator PU.1. Reduction in PU.1 leads to lower expression of phagocytic proteins and abnormal synaptic pruning in males. A failure to prune non-functional connections during this critical period of hippocampal development leads to inefficient wiring grid that persists into adulthood and impairs normal hippocampal function in male mice. In support of this hypothesis we show that BDS causes male-specific retention of abnormally shaped spines and impairs the ability of male but not female microglia to phagocytose synaptosomes ex vivo. Corticosterone levels are higher in P14 males compared to female BDS littermates and administration of the glucocorticoid antagonist, RU486, restores normal phagocytic activity and HP-dependent function in BDS male mice. Microglia from BDS male, but not female, mice show reduced PU.1 levels and lower expression of several genes implicated in cell migration and phagocytic activity. Finally, mice with reduced levels of PU.1 in microglia (PU.1-hets) show increased anxiety and abnormal HP- function that resemble behavioral abnormalities seen in male BDS mice. Work in aim 1 will test whether PU.1- hets show similar developmental and behavioral abnormalities to those seen in BDS male mice and whether they are more resilient or sensitive to the developmental and behavioral consequences of BDS. Studies proposed in aim 2 will clarify the role that elevated corticosterone and activation of the GR play in regulating PU.1 levels and microglial function. In aim 3 we will test whether lower levels of microglial PU.1 are seen in other mouse models of early life stress. If successful, this application will define a novel role for PU.1 in mediating developmental and behavioral abnormalities in several paradigms of early-life stress in the mouse.